Why can a shaft key change the meaning of a rotor balance certificate?

The short answer

A key adds rotating mass at a particular radius. A balance result using a full key, half key or no key therefore allocates that mass differently between shaft and attached hub. Matching the convention matters when a wind generator shaft is coupled to another rotating component.

Understand the allocation

A keyed assembly contains the final installed key, but its two components can be balanced separately. The balancing convention specifies how key mass is represented in those separate operations. It does not mean that the final machine should be assembled with a shortened or partly missing key. An unspecified convention can leave one portion of the key mass omitted or counted twice in component balancing.

Read both sides of the interface

Check the shaft’s marking or report together with the coupling-hub balancing instruction. ABB motor documentation provides an example in which half-key balancing is stated explicitly; that is evidence of a convention, not a MARSTON manufacturing claim. Do not assume an unmarked replacement hub follows the same convention or copy ABB colour markings to another supplier. The actual interface needs compatible component-specific documentation.

Resolve a mismatch before accepting the assembly

Ask the generator and coupling suppliers to state the balance convention, relevant key dimensions and the assembled boundary covered by their results. Retain the distinction between key geometry needed for torque transmission and key mass representation needed for balance. If the conventions differ, obtain an approved resolution rather than remove material or alter the operating key. A consistent convention supports mass accounting; it does not independently prove acceptable residual unbalance at every speed or certify the complete wind rotor. Also confirm whether the actual operating key length matches the interface design. A consistent balancing convention cannot correct a wrongly specified key, and the convention must never be interpreted as an assembly instruction to omit material.